Treatment of mammalian semen to achieve a higher proportion of fertility and/or a higher proportion of one gender over another in artificial insemination can be advantageous. For example, a dairy herd would obtain economic and genetic herd quality benefit from an increase in numbers of cows pregnant at any given time and/or birthing a higher percentage of heifers relative to bulls. In such a situation, replacement animals for the herds are produced more efficiently. In addition, especially with low-beef value animals such as Holsteins, the expense of bull calves, and the potential cruelty these animals face when used in veal production is reduced.
The availability of replacement female animals born at the dairy farm eliminates the need to import replacements and the attendant risk of disease introduction into a herd. Additional advantages are found for businesses housing elite sires that produce dairy bull semen. Since these bulls are evaluated, i.e. “sire-proofed,” for genetic quality—through their daughters, an elite bull can be brought into semen production more quickly if he produces daughters more quickly and often. This speeds improvement of the sire genotype, with the attendant competitive advantage. This further produces a savings in feed, vet, and other costs associated with bull farming. It also accelerates the improvement of the genetic base of dairy herds using semen from these processors, with the attendant economic savings to dairy farmer and semen processor alike.
In addition, achieving good fertility by increasing the quality of sperm used in artificial insemination is considered to be the single greatest determinant of the success or failure of dairy farms. Since “open” or non-pregnant cows do not lactate and are therefore not productive, they decrease profit. Consequently, any increase in fertility is considered worthwhile. Similar situations exist for other types of animals raised for dairy such as goats, sheep, cattle, buffalo, camels, swine, etc.
In another example, increased sperm quality can lead to improvement and/or expansion of a particular population of animals. For instance, sperm collected from elite race horses or other champion animals, such as cattle or other livestock and particular breeds of dogs and cats, is commonly used for artificial insemination to increase the probability of maintaining particular features in the gene pool. Sperm quality is particularly important in the breeding programs directed to exotic and endangered animals where the number of captive individuals is limited. Here, the ability to increase overall birth rates, thereby increasing the potential for rapid expansion of the population, is critical for success.
In another example, the personal suffering and costs associated with human infertility can in many cases be reduced through increasing sperm quality. Couples whose infertility is caused by low sperm count or poor sperm motility can benefit by increasing the number of intact and viable sperm that result after the washing and preparations steps needed prior to intrauterine artificial insemination (IUI) or intracytoplasmic sperm injection (ICSI).
With respect to gender bias, the suffering and costs of human sex-linked diseases can be reduced through birth of females in affected human families. Female births are the only way to eliminate over 300 X-linked diseases, many of which shorten and impair quality of life and create staggering medical costs. Currently, the costs and suffering associated with these diseases can be decreased through pre-implantation genetic diagnosis. In this process, eggs are harvested by laparoscopy following injections of hormones and fertility drugs. Eggs are fertilized in vitro and, after embryos have reached sufficient size, a single cell is microdissected from each embryo for genetic analysis. A suitable unaffected female embryo is chosen for implantation. Alternatively, sperm is collected and treated with mutagenic dye in preparation for fluorescent activated cell sorting (FACS). X-bearing sperm are obtained, however, they are so damaged that the sperm nucleus must be injected into an isolated egg in vitro using intracytoplasmic egg injection. Embryos are then cultured and implanted in recipients. Both of these techniques are expensive and raise unresolved questions about the effect of exposure to DNA-binding dyes and laser light, with respect to their cytotoxicity and mutagenic potential (Downey et al. (1991) J. Histochem. and Cytochem. 39: 485-489; Durand and Olive (1982) J. Histochem. and Cytochem. 30:111-116).
The scientific literature describes several methods for achieving gender bias through treatment of mammalian semen. They differ in process; some involve physical separation of sperm while others do not. They also differ at point of application; to sperm, to female mammals, to clutches of eggs in egg-laying animals. What they share in common is that they cannot be applied effectively on-site. In addition, with the exception of the instant invention, there is no technology readily used for insemination of cows. Fertility issues with other technologies restrict their use to virgin heifers, which are less stressed and therefore have higher fertility than cows that have experienced the stress of lactation.
For example, several methods are reported for generating sex bias by physical separation of sperm, all of which involve complex laboratory manipulations and equipment. Fluorescence activated cell sorting (FACS) resolves sperm into X (female) and Y (male) bearing pools, after cell labeling with mutagenic DNA-binding dyes to reveal chromosome content (Abeydeera et al. (1998) Theriogenology 50: 981-988; Cran and Johnson (1996) Human Reproduction Update 2: 355-363). Methods of artificially biasing the sex of mammalian offspring through physical separation have also included methods based upon density sedimentation of spermatozoa (e.g. Brandriff et al. (1986) Fertil. Steril. 46:678-685) and by separating the population of spermatozoa into fractions that differ by the sex-linked electrical charge resident thereon (U.S. Pat. No. 4,083,957). Methods have also been described that rely on mechanical sorting of sperm by sex-type. U.S. Pat. No. 5,514,537, for example, uses a column packed with two sizes of beads. The large beads are of a diameter so that the smaller beads will fall between the interstices created between the larger beads. Then the interstices between the smaller beads allow Y-bearing sperm to enter them while the X-bearing sperm are excluded, thereby effecting separation of the two subpopulations. Separation based on immunological methods and cell surface markers have also been proposed (Blecher et al. (1999) Theriogenology 52: 1309-1321). In another example, U.S. Pat. No. 3,687,806 discloses an immunological method for controlling the sex of mammalian offspring using antibodies that react with either X-bearing sperm or Y-bearing sperm which uses an agglutination step to separate bound antibodies from unaffected antibodies. U.S. Pat. No. 4,191,749 discloses a method for increasing the percentage of mammalian offspring of either sex by using a male-specific antibody coupled to a solid-phase immunoabsorbant material to selectively bind male-determining sperm while female-determining sperm remain unbound in a supernatant. U.S. Pat. No. 5,021,244 discloses a method for sorting living cells based upon DNA content, particularly sperm populations to produce subpopulations enriched in X-bearing sperm or Y-bearing sperm by means of sex-associated membrane proteins and antibodies specific for such proteins.
Some methods have combined various aspects of the immunological and mechanical separations such as U.S. Pat. Nos. 6,153,373 and 6,489,092 which use antibodies coupled to a magnetic particle for separation of sperm.
Separation based on a miniscule size difference between X- and Y-bearing sperm has also been attempted (Van Munster et al. (1999) Theriogenology 52: 1281-1293; Van Munster (1999) Cytometry 35: 125-128; Van Munster 2002 Cytometry 47: 192-199).
In addition, sex bias without physical separation of sperm into X and Y bearing classes has been described. For example, stress (Catalano et al. (2006) Human Reproduction 21: 3127-3131), good or poor physical condition (Trivers and Willard (1973) Science 179:90-92), feed composition (Alexenko et al. (2007) Biol. Reprod. 77:599-604), temperature (Crews (1996) Zoological Science 13: 1-13) and other factors (Wedekind (2002) Animal Conservation 5:13-20) have been shown to affect offspring sex ratio.
Lechniak (2003, Reprod. Dom. Anim. 38:224-227); has also shown that time-based production of a sex bias sexing of semen can occur when semen is held for various times before use in insemination for in vitro fertilization. However, the exact time course of activation of sperm from its dormant state at the time of collection, through its various metabolic states of fertility, until the sperm finally become infertile and atophied, varies between different species of mammals, and also between different—individuals of the same species, and even between ejaculates obtained from the same individual animal.
This large degree of variability in time course from semen samples collected from the same individual led those skilled in the art to conclude that a fertile semen sample having a gender bias could not be reliably obtained simply by processing a sample after a standard period of time after collection of the semen sample. Therefore, there is a need in the art to develop a time based assay on which one can reliably depend to provide a semen sample containing sperm which have a desirable trait, such as a fertile, gender biased, semen sample. Ideally, the assay could be performed on site.